Although not distributed as widely as iron or copper in the biosphere, manganese plays an essential role in numerous enzymatic reactions. The most significant activity is its unique application in the photosynthetic water oxidation reaction where the oxygen evolving complex (OEC) catalyzes the conversion of two equivalents of water to dioxygen, 4 protons and 4 electrons. Four manganese and two calcium ions are optimal activity. Other Mn enzymes are involved in oxygen metabolism (e.g., catalase or superoxide dismutase) or use oxygen metabolites to cause chemical transformations (e.g., Mn ligninase degradation of woody plants). We propose a structure/reactivity approach to evaluate three activities associated with manganoenzymes. We will prepare monomeric and trimeric structural models to evaluate the mononuclear/trimer nuclearity proposal for metal organizations in the OEC and mixed metal complexes to evaluate the suggestion that the Mn ensemble may be better described as a Mn/Ca cluster. We will also prepare new dinuclear materials and develop mineral synthetic methodologies to rationally prepare dimers with desired bridging ligands. With these compounds in hand, we will focus on defining the interesting reactivity of dimeric materials with hydrogen peroxide (catalase mimic), hydrazine, acid (water oxidation), dioxygen, and other small molecule oxidants. We will also explore the reactivity of mononuclear compounds thought to be the active catalyst in lignin degradation and define conditions where manganese acts as a one electron oxidant/reductant (e.g., oxidative decarboxylation reactions). This mix of emphasis on new structural definition combined with elucidation of biologically relevant redox reactions will provide biologists, biophysicists and chemists the necessary basic information to develop reasonable structural and mechanistic proposals for manganese catalyzed metabolism of dioxygen and its reduced forms.